<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>小球变速运动</title>
    <style type="text/css">
        body {
            margin: 0;
        }

        #ball {
            width: 100px;
            height: 100px;
            background-image: radial-gradient(circle at center, red, black);
            border-radius: 50%;
            position: absolute;
        }

        #board {
            margin: 10px auto;
            width: 600px;
            height: 100px;
            outline: 5px solid black;
            position: relative;
        }
    </style>

</head>nieyi geiwo zheixe a nieyigeiwo zhexie de hua  ni P
<body>

<section id="board">
    <div id="ball"></div>
</section>

</body>
</html>